In an Asymmetrical Digital Subscriber Lines (ADSL) the data is encoded into constellation points by multiple (one for each carrier) constellation encoders. A typical ADSL system uses DMT based modulation. Another term for this type of modulation system is referred to as Orthogonal Frequency Division Multiplexing (OFDM) system. The complex values of the constellation encoder outputs are the inputs to an inverse fast Fourier transform (IFFT). A typical ADSL transmitter 10 after modulation uses several stages of digital filtering 12 followed by digital to analog conversion (DAC) 13, one or more stages of analog filtering 15, line driver 16 and analog front end 17 including hybrid 18 connected to the line as shown in FIG. 1. At the receiver, the reverse process is performed; i.e., the received signal is amplified and filtered prior to analog to digital conversion, further processed by digital filters and a FFT is used to recover the carriers. In the receiver the multiple carriers are then demodulated in multiple constellation decoders (one for each carrier), recovering the original data.
At the transmitter after modulation (after IFFT, cyclic prefix addition, etc. 11), the digital filtering 12 typically consists of several interpolation filters to match the digital to analog converter (DAC) 13 sampling rate. Apart from interpolation, these filters perform most of the spectral shaping to comply with a given Power Spectral Density (PSD) mask. A PSD mask is a template that specifies the maximum PSD allowable for a line code. PSD masks are used as both guidelines for the design and implementation of a DSL technology as well as for crosstalk modeling to simulate and benchmark performance. In addition, for a frequency division duplexing (FDD) system, these filters also perform the band split. The analog filtering 15 following the DAC 13 provides the desired out-of-band attenuation for transmit noise while the line driver adjusts the transmit power to adhere to a desired total transmit power.
All analog and digital filters have a finite amount of pass-band ripple and/or group delay distortion associated with them. The analog front-end (transformer, coupling capacitors, etc.) and the impedance mismatch between the hybrid and the line (channel) may also distort the signal. Also the amplitude/phase response of the analog filters are sensitive to process variations and hence the actual hardware can vary slightly from the nominal values and also between chip to chip. Moreover in certain ADSL codec designs, significant analog power/area savings and/or improved noise performance may be achieved by reducing the amount of analog filtering in the transmitter. Another advantage of reducing the analog filtering is that the group delay distortion in the transmit path may also be reduced. To do this and yet achieve a given stop band attenuation, the analog filter may have to cut into the upper part of the transmit spectrum. All of these could reduce upstream rates and/or make equalization of the upstream channel difficult.